Techniques for subjecting an image to desired deformation are conventionally known. For example, when correcting a distortion of an image created by photography, a process for deforming the image is performed. For example, NPL1 describes a technique for correcting a distortion of an image photographed using a fisheye lens. In addition, NPL2 presents an example of a method of correcting a distortion created by a fisheye lens. Various other image deformation processes are available including an image deformation process for correcting a distortion of an image created by the lens of a camera and a process for performing geometric correction on a photographed image in order to superimpose the image on a map.
Such image deformation processes geometrically associate respective pixels in an image after correction to positions in the image prior to correction. There are various correspondence models and distortion measurement methods used for image deformation processes according to characteristics of a photographic system or a distortion.
When a position in an image prior to correction to which a pixel after correction is associated exactly overlaps a position of a discretely existing pixel, a pixel value of the image prior to correction can be applied without modification to a pixel value of the image after correction. However, generally, a position to which a pixel after correction is associated by geometrical correspondence does not overlap with a position of a pixel in the image prior to correction and falls on a midway position between pixels. Therefore, a pixel value of a pixel after correction is obtained by an interpolating process from a pixel value of a pixel in a vicinity of the midway position.
Various methods are being proposed as interpolation processing methods. For example, NPL3 describes several interpolation methods.
Typical examples of interpolation methods include three methods known as “nearest neighbor interpolation”, “linear interpolation”, and “cubic interpolation”. Among the three methods, “cubic interpolation” achieves the highest accuracy. An interpolation method capable of achieving even higher accuracy is called Lanczos-n interpolation.
In addition, a phase restriction correlation method described in NPL4 is available as a method of evaluating accuracy of an image deformation process such as distortion correction involving an interpolation process. With the phase restriction correlation method, a deviation of an image after deformation from an original image can be measured to an accuracy of a level of subpixels that represent a fine division of a space between pixels.